1. Field of the Invention
Embodiments of the present invention relate generally to optical communication systems and, more particularly, to a fiber lens assembly and an optical device having a fiber lens assembly.
2. Description of the Related Art
In optical fiber-based communications systems, optic fibers are coupled to various signal processing devices for switching, attenuation, and broadcasting functions. These devices include wavelength selective switches (WSSs), optical add-drop multiplexers (OADMs), dynamic gain equalizers (DGEs) and wavelength selective routers, among others.
An optical fiber is a cylindrical dielectric waveguide that transmits light along its axis by the process of total internal reflection. The fiber consists of a core surrounded by a cladding layer, where the core makes up the light-transmitting portion of the fiber and is generally on the order of about 10 micrometers in diameter. Relative to other optical components of a WSS, OADM, or DGE, which are generally free-space optical components, the fiber core is orders of magnitude smaller. Because the fiber core has such a small diameter, precise placement of the fiber in relation to the other optical elements of a device is necessary for the fiber to be optically coupled thereto. For example, if the optical fiber for an output port is displaced by only a few micrometers from the optical axis of an outgoing light beam, a substantial portion of the outgoing light beam will not be incident on the core of the fiber, resulting in significant signal loss. In fact, if the displacement between the fiber and the optical axis of the outgoing light beam is greater the radius of the fiber core, none of outgoing signal will reach the output fiber. Therefore, even a very small positional displacement of an optical fiber, e.g., five micrometers, can produce serious signal loss. Misalignment between the optical axis of an outgoing light beam and a fiber may occur when the fiber is initially coupled to an optical device, or as a result of drift. Drift is generally caused by the small relative motion between the components of an optical switching device that occurs due to thermal expansion and/or contraction of the optical switching device during operation.
Another source of signal loss in optical switching devices is the angular displacement between the ideal alignment of a fiber coupled to an optical switching device and the actual alignment of the fiber. As with positional displacement, angular displacement may be introduced during installation of the fiber and/or via thermal expansion/contraction of the optical switching device during operation. Collimated light beams, which are generally used in WSSs, OADMs, and other optical switching devices, are particularly sensitive to angular misalignment issues.
FIG. 1A schematically illustrates the sensitivity of a collimated light beam to angular misalignment of a fiber lens assembly by comparing fiber lens assemblies 100A and 100B. Fiber lens assembly 100A is centered on and aligned parallel to an ideal optical axis 103. Fiber lens assembly 100A includes an optical fiber 101 and a collimating lens 102, and produces a collimated light beam 110A. Because fiber lens assembly 100A has ideal angular and positional alignment with ideal optical axis 103, collimated light beam 110A is coincident with ideal target region 105 as the beam crosses an image plane 104. Ideal target region 105 corresponds to a critical region or optical element of an optical switching device that requires precise alignment of an incident light beam, such as an aperture, diffraction grating, mirror, or optical steering device. Misalignment of the beam with ideal target region 105 results in signal loss and/or poor performance of a signal processing device coupled to fiber lens assembly 110A.
Fiber lens assembly 100B is substantially identical to fiber lens assembly 110A, except that fiber lens assembly 100B produces a collimated light beam 110B, which is centered on optical axis 103B. Optical axis 103B has an angular displacement θ from ideal optical axis 103, where angular displacement θ may be caused by imprecise installation of fiber lens assembly 100B, or by mechanical drift, thermal expansion, etc. As shown in FIG. 1A, although optical fiber 101 of fiber lens assembly 100B is positionally centered on ideal optical axis 103, angular displacement θ of fiber lens assembly 100B causes collimated light beam 110B to cross image plane 104 at region 105B instead of ideal target region 105. In this example, angular displacement E results in more than half of collimated light beam 110B missing ideal target region 105. In general, a small angular misalignment of fiber lens assembly 100B can produce large signal loss, particularly when the distance 106 between collimating lens 102 and image plane 104 is much larger than the diameter 107 of the collimated light beam.
Angular misalignment and/or drift of optical elements can also produce high losses at an output port of an optical switching device. FIG. 1B schematically illustrates a typical output port 120 of an optical switching device. Output port 120 includes an optical fiber tip 121 and a focusing lens 122. Output port 120 is configured to receive a collimated light beam at focusing lens 122 and focus the beam onto optical fiber tip 121. Beam 123 is a collimated light beam incident on output port 120 that is aligned with an ideal optical axis 124, and therefore is completely focused onto optical fiber tip 121 without any signal loss. Beam 125 is a collimated light beam that has a small angular misalignment θ125 with ideal optical axis 124. As a result, the focal point of beam 125 is altered as shown by a displacement H from ideal optical axis 124, and is not directed entirely onto optical fiber tip 121, resulting in significant signal loss. For larger angular misalignments, displacement H may be greater than the radius of optical fiber tip 121, e.g., five or more micrometers, resulting in essentially 100% signal loss. Because focal length L of optical output port 120 is typically several orders of magnitude greater than the diameter of optical fiber tip 121, even small angular misalignment of an incident light beam with optical output port 120 will produce a relatively large displacement H and an associated signal loss.
Accordingly, there is a need in the art for a robust means of coupling an optical fiber to signal processing devices used in communications networks that is less sensitive to positional and angular alignment over prior art means.